Thermal transfer systems for thermal imaging utilize a recording method in which a donor sheet, having a colorant (i.e., dye or pigment) layer thereon, and a receptor sheet are brought into contact and heated in an imagewise manner, as with a thermal print head, laser, etc. The image-distributed heat source, such as a thermal print head, directly contacts the backside of the donor sheet. A thermal print head contains small electrically heated elements that can be selectively heated, thereby transferring colorant from the donor sheet to the receptor sheet and forming a desired image. This imaging process can involve either mass transfer of colorant in a binder or state-altered transformation of a dye, as by melting or sublimation of the colorant. In a mass transfer process, the colorant, e.g., dye or pigment, is dispersed within a binder and both the dye and its binder are transferred from a donor sheet to a receptor sheet. In a dye transfer process, the colorant (present on the donor with or without a binder) is transferred without binder by melting, melt-vaporization, propulsive ablation, sublimation, or vaporization to a receptor sheet where the colorant adheres to a receptor sheet or diffuses into an image-receiving layer.
Foreign substances, such as dust, can create areas of noncontact between the donor and receptor sheets or between the donor sheet and print heads, for example. Such noncontact areas adversely effect the transfer of an image. For example, a single particle of dust can easily get trapped under a print head and streak an image. This detrimental effect can occur whether the transfer occurs by mass transfer or dye transfer. Generally, foreign substances such as dust are attracted to the donor and/or receptor sheets as a result of electrostatic attraction to built-up electrical charges.
There is a growing interest in the use of antistatic materials and coatings to solve the problems created by the build-up of electric charges, i.e., "static electricity" in various fields of technology, such as the photographic, electronics, and magnetic recording industries. Antistatic materials, i.e., antistats, are electrically conductive materials that are capable of transporting charges away from areas where they are not desired. This conduction process results in the dissipation of the static electricity. In certain situations this results in a decrease in the buildup of dust.
A typical antistatic layer comprises an organic or inorganic conductive material in a binder. The layer dissipates electrical charges by the conduction of charged particles, which can be either ions or electrons. Ionically conductive antistatic coatings are thought to act as electrolytic solutions through which ions are transported under the influence of an electric field. They are typically salts or hydrophilic chemicals that are applied to the surface of an article. As such, they threaten contamination and/or corrosion of material, e.g., electronic components, and may interfere with the function of materials with which they come in contact. Furthermore, being hydrophilic or water soluble, they lack permanence when in contact with water. The use of ionically conductive coatings is especially difficult in applications in which the surface coatings must be in contact with air. For example, low friction layers, dye donor layers, and dye receptor layers must typically not be overcoated by materials that interfere with their function. Also, the function of ionically conductive antistatic coatings is dependent upon humidity. At low humidity, the coating is not sufficiently conductive to provide rapid dissipation of triboelectrically generated charges, i.e., charges resulting from friction-causing events such as unwinding and handling. Furthermore, at high humidity the coating can become soft, sticky, and can undergo a large volume change.
Preferable antistats for many applications are those that conduct electrons by a quantum mechanical mechanism rather than by an ionic mechanism. This is because antistats that conduct electrons by a quantum mechanical mechanism are generally effective independent of humidity. They are suitable for use under conditions of low relative humidity, without losing effectiveness, and under conditions of high relative humidity, without becoming sticky. Furthermore, such electronically conductive antistatic coatings remain effective when overcoated by, for example, a dye donor layer or dye receptor layer. A major problem, however, with such electron-conducting antistats is that they generally cannot be provided as thin, transparent, lightly colored or relatively colorless coatings by solution coating methods.
Metal particle, metal oxide particle, or carbon black dispersions can be used to provide electronically conductive coatings via solution deposition methods; however, such coatings tend to be darkly colored and opaque. This is generally not desirable for use in thermal transfer systems. Although there have been many attempts to do so, such as by using defect semiconductor oxide particle dispersions and conductive polymers, there has been very little success in depositing thin, transparent, lightly colored or relatively colorless electronically conductive antistatic coatings.